High intensity discharge lamps of the mercury vapor or metal halide types generally comprise a quartz or fused silica arc tube mounted within a glass outer envelope or jacket having a screw base at one end. The arc tube contains the discharge medium such as mercury and metal halide, along with an inert gas such as argon for starting purposes. The arc discharge takes place between thermionic main electrodes which are sealed into the ends of the arc tube. In order to facilitate starting of the lamp, it has been common practice to provide also an auxiliary starting electrode which is sealed into an end of the arc tube adjacent one of the main electrodes. The auxiliary electrode may be a tungsten wire projecting into the arc tube discharge space and connected through a current-limiting resistor to the main electrode remote from it.
A starting electrode is very useful to ease the burden on the ballast in starting the lamp but it has definite drawbacks. Metal halide salt condensed in the region about the starting electrode and the adjacent main electrode may be subjected to electrolysis leading to accelerated failure of the seal at the end of the arc tube having the starting electrode. U.S. Pat. No. 3,226,697--Green, 1965, provides a solution to this problem in the form of a thermal switch arranged to short-circuit the starting electrode to the adjacent main electrode when actuated by heat from the arc tube after the lamp has started and warmed up. While this is a good practical answer which has been widely adopted, it is not perfect. The delay in closing the thermal switch while the arc tube warms up is a definite factor in reducing maintenance and shortening lamp life, particularly in applications where the lamp is turned on and off frequently. Furthermore the starting electrode complicates manufacture and makes a wider seal necessary which it may be desired to avoid.